1. Field of the Invention
The present invention relates to a vehicle motion control system for controlling motion of a vehicle, and more particularly to the control system for maintaining vehicle stability by controlling a braking force applied to each wheel of the vehicle irrespective of depression of a brake pedal in the vehicle motion including a turning motion of the vehicle.
2. Description of the Related Arts
As a system for controlling a vehicle motion characteristic, especially a turning characteristic of the vehicle, an apparatus for controlling a difference between the braking force applied to right and left wheels to directly control a turning moment has been noted, and it is now on the market. For example, Japanese Patent Laid-open Publication No.9-301147 discloses a motion control apparatus which estimates an amount indicative of a condition of the vehicle in motion when the vehicle turns, and controls a hydraulic braking pressure control apparatus so as to correct a yaw moment of the vehicle to make the vehicle in motion to be stable, when the amount indicative of the condition of the vehicle in motion exceeded a threshold level for starting the control. In that publication, in order to change a range of the amount of the vehicle motion for starting the control in accordance with a coefficient of friction, is proposed the motion control apparatus, wherein the lower the coefficient of friction is, the smaller the threshold level for starting the control is set.
In the vehicle motion control system as described above, various sensors for detecting signals indicative of the condition of the vehicle in motion have been disposed. For example, Japanese Patent Laid-open Publication No.5-314397 discloses a yaw rate sensor for detecting a yaw rate of the vehicle. In this Publication, a zero point signal of the yaw rate sensor is renewed to provide such a relationship that each of positive or negative value of steering angle within a certain time period and each of positive or negative value of yaw rate within a certain time period will coincide with each other, under a certain running condition of the vehicle.
According to the apparatus for processing the sensor signal as described in the Publication No.5-314397, the renewal of the zero point may cause an error, when controlling the vehicle motion. Therefore, it is preferable not to start the vehicle motion control, when the zero point is renewed. In an apparatus having a lateral acceleration sensor for detecting the lateral acceleration of the vehicle, the center of gravity of the vehicle moves, when the vehicle runs on a rough road, for example, so that a distance will be made between the center of gravity and a position of the vehicle on which the center of the sensor is mounted, to cause an error in the detected lateral acceleration. In practice, an actual acceleration (Gyo) is calculated in accordance with the following equation: EQU Gyo=Gya-L.multidot.d.gamma./dt
where "Gya" is a value detected by the lateral acceleration sensor, "L" is the distance between the center of gravity and the position of the vehicle on which the center of the sensor is mounted, and ".gamma." the yaw rate. Thus, the error of (L.multidot.d.gamma./dt) is caused. In particular, when a vehicle slip angle for use in a steering control by braking is calculated, the error in the output of the lateral acceleration sensor is accumulated, thereby to cause a relatively large error. Therefore, it is preferable to set a gain for starting the control to be less sensitive, such that the vehicle motion control will not start, when the error caused by the lateral acceleration sensor is estimated.
In the motion control apparatus as disclosed in the Publication No.9-301147, the threshold level for starting the control is corrected in response to the coefficient of friction of the road, but it is not corrected in advance by estimating the error caused by a monitor like the sensor as described above.